Everytime you access a website a server is serving you files. Apache (and most web servers) keep logs of this. With Apache defaults you get IP address, the route accessed, and the User-Agent of the user. This is rudimentary information, but if you have these logs from multiple sites, it's pretty easy to roughly track someone. Tracking images in emails use this same principle, a unique link to krick.png is put in an email sent to you, and if it gets served by the server (shows up in the access logs) it's pretty reasonable to assume that you read the email.
If you want to see a simplified version of what this log looks like, run 'python -m SimpleHTTPServer' and visit localhost:8000.
Have you even read my message? Or the thread you are answering to for that matter? The question is not how website owner knows I visited his website, that much is pretty obvious, but if it is the case that server-side tracking somehow allows to use GoogleAnalytics as well (that is, to notify Google from server side who has visited their website) and if this is the case — how does it exactly work. Because that's what JupiterMoon seems to be claiming.
If you want to see a simplified version of what this log looks like, run 'python -m SimpleHTTPServer' and visit localhost:8000.